This invention relates to cathode ray picture tubes, and is addressed to means for abating the low frequency magnetic radiation that emanates from the beam-deflecting yoke of such tubes. More particularly, the limiting is applicable to the very low frequency magnetic radiation of the vertical deflection field of yokes used in high-resolution, single-beam, monochrome visual display terminals (VDT's).
The present invention had its origin in the concern over the possible detrimental effects of the magnetic field component of electromagnetic radiation. Testing for electromagnetic emission in VDT's is described in a booklet published by the National Board for Measurement and Testing (MPR) of Sweden entitled "Test Methods for Visual Display Units: Visual Ergonomics and Emission Characteristics," MPR 1990:8 1990-1991, Boras, Sweden, known as standard MPR-2. Electromagnetic radiation is also a subject of study by the IEEE Working Group P-1140 on a Standard for Measurement of Electromagnetic Near Fields (5 Hz to 30 MHz).
The yoke used in monochrome VDT's is an electromagnetic device that causes a single beam to scan a raster on a CRT viewing screen in the horizontal and vertical directions. Essentially, a yoke consists of two pairs of coils, one deflecting the electron beam in the horizontal direction, and the other in the vertical direction. The two pairs appear as dual radiating magnetic dipoles.
In producing the respective deflecting fields, the coils also produce non-deflecting fields which constitute undesired magnetic fields which radiate beyond the perimeter of the tube and the cabinet in which the tube is enclosed. To cancel the undesired fields, two additional coil pairs have typically been placed in close proximity to the yoke. The additional coil pairs in effect cancel out the flux of the undesired fields and the resulting external radiation. The cancellation achieved however is at the cost of greater circuit complexity and the requirement for additional parts. Also, the magnetic influence of the additional coil pairs can degrade the performance of the yoke.